This disclosure relates generally to tracking systems that use magnetic fields to determine the position and orientation of an object, such as systems used for tracking instruments and devices during surgical interventions and other medical procedures. More particularly, this disclosure relates to a system and method for electromagnetic tracking that utilizes a compact coil arrangement.
Tracking systems have been used in various industries and applications to provide position information relating to objects. For example, electromagnetic tracking may be useful in aviation applications, motion sensing applications, and medical applications. In medical applications, tracking systems have been used to provide an operator (e.g., a physician) with information to assist in the precise and rapid positioning of a medical device located in or near a patient's body. In general, an image may be displayed on a monitor to provide positioning information to an operator. The image may include a visualization of the patient's anatomy with an icon on the image representing the device. As the device is positioned with respect to the patient's body, the displayed image is updated to reflect the correct device coordinates. The base image of the patient's anatomy may be generated either prior to or during the medical procedure. For example, any suitable medical imaging technique, such as X-ray, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and ultrasound, may be utilized to provide the base image displayed during tracking. The combination of the base image and the representation of the tracked device provides positioning information that allows a medical practitioner to manipulate a device to a desired position and/or associate information gathered to a precise location.
To determine device location, tracking systems may utilize a method of electromagnetic (EM) field generation and detection. Using this method, at least one magnetic field is generated from one or more EM sensors (e.g., EM field generators or transmitters), and the magnetic fields are detected by one or more complementary EM sensors (e.g., EM receivers). In such a system the EM field may be detected by measuring the mutual inductance between the EM sensors and the complementary EM sensors. The measured values are processed to resolve a position and/or orientation of the EM sensors relative to one another. For example, an electromagnetic tracking system may include an EM sensor mounted at the operative end of a device and a complementary EM sensor fixed in a known position. When the EM sensor generates a magnetic field, a voltage indicative of the mutual inductance may be induced across the complementary EM sensor. The signal may be sensed and transmitted to a processor for processing. Processing may then use the measured voltage signal indicative of mutual inductance to determine the position and orientation of the EM sensors relative to one another (e.g., the X, Y and Z coordinates, as well as the roll, pitch and yaw angles).
Generally electromagnetic tracking systems contain EM sensors that consist of an array of one or more EM field generators and an array of or more EM receivers. To provide for more accurate device tracking, various arrangements of EM sensors around a tracking area have been used. For example, four EM sensors may be located at the corners of a rectangular region. In this configuration, a single EM sensor may act as act as an EM field generator generating magnetic fields that are sensed by the complementary EM sensor acting as an EM receiver. A processor may then receive the signals indicative of the detected magnetic fields and triangulate the position and/or orientation of the EM sensors.
Although the previously described methods may provide sufficient accuracy, there are several instances in which the sensors located about the periphery of a region in a single plane do not provide a sufficient estimate of position and/or orientation. For example, when four EM receivers are located in a single plane (e.g., on top of a surgery table) processing may be unable to accurately track the distance of an EM field generator above or below the plane. Further, even if the distance is determined, it may be difficult (if not impossible) for processing to determine on which side of the plane the EM field generator is located. For example, the same mutual inductance measurements may be present for an EM field generator located above the plane at a given position (x, y, z) and an EM field generator located below the plane at a given position (x, y, −z). This is because at both locations the EM field generator is the same distance from the EM receivers and, therefore, the mutual inductance measurements and their ratios used for triangulation are the same. Thus, processing is unable to resolve on which side of the plane the EM field generator is located.
In addition, some tracking system applications require compact EM sensors. Some EM sensors that contain a number of coils can be quite large. Industry standard coil architecture (ISCA) EM sensor assemblies are advantageous for these applications because they are small. However, the ISCA transmitter and receiver each comprise three approximately concentric and approximately orthogonal coils. Also, the ISCA coils must be individually characterized. When an ISCA transmitter or receiver needs to be mounted in the tip of a surgical instrument, space is restricted, making the use of a three coil device difficult.
Accordingly, there is a desire to provide an EM tracking system, wherein the EM sensors include at least one compact coil array.